High efficiency nitrogen fixation bacteria were isolated from rice soil. The most effective strain G3 was selected（1.875 μmol·ml^-1·h^-1） by acetylene reduction method. Based on the morphological characters, G3 strain was gram negative bacteria. The optimal growth of G3 was studied. The experimental results showed that the optimal temperature and pH were 35℃ and 7, respectively. The optimal concentration of NaCl was 1%, and optimal NH₄⁺ was 1 mmol/L.